Mass spectrometers are used to identify and quantitate compounds. Mass spectrometers analyse compounds by the mass to charge ratio of ions formed of the molecules of such compounds or the fragments of such molecules. Mass spectrometers generally have ion sources, which provide ions of the compounds for analysis. One form of ion source is an atmospheric pressure ionization (API) sources. An API source is, as its name suggests, an ion source that creates ions at approximately atmospheric pressure. These ions are directed to substantially closed sections of the mass spectrometer operating at low pressure or vacuum.
API sources suitable for generating ions from solutions include electrospray, atmospheric pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) sources, all of which involve the formation of an aerosol from the solution. Electrospray sources form the aerosol by means of an electrical field created between an inlet capillary through which the solution is introduced and a counter electrode disposed downstream of the exit of the capillary. This electrical field also results in the ionization of at least some of a sample dissolved in the solution. APCI and APPI ion sources form the aerosol by means of a nebulizer, usually a concentric flow pneumatic nebulizer, and further comprise additional means for ionizing sample molecules comprised in the aerosol. These additional means may comprise a corona discharge (APCI sources) or a beam of photons (APPI sources). A nebulizer may also be used in electrospray sources to increase the maximum solution flow rate that the source can accept. Ionization may also be effected by a combination of some or all of the methods described.
API sources, which produce an aerosol comprising electrically charged droplets of the solution, are produced in a region containing gas at approximately atmospheric pressure. The charged droplets may comprise solvated ions characteristic of the sample dissolved in the solution. Droplets and solvated ions are sampled from the aerosol into a region of lower pressure through a small orifice or capillary tube, usually along a sampling axis inclined to the central axis of the aerosol. At least some of the ions entering the region of lower pressure are subsequently transmitted to a mass analyser through a sequence of vacuum chambers of progressively reducing pressure. These vacuum chambers usually comprise ion guides of various types. Mass analysers used in conjunction with these ion sources include linear quadrupoles, quadrupole, cylindrical and “Kingdon” ion trap analysers, magnetic sector analysers, ion cyclotron resonance analysers (ICR or FTMS analysers), time-of-flight analysers, or combinations of these analysers for use in tandem (MS/MS) apparatus. API ion sources are also used in ion mobility spectrometers, including field asymmetric ion mobility spectrometers (FAIMS) and in mass spectrometers comprising an ion mobility stage as well as more conventional mass filters or analysers. The charged droplets present in the aerosol may be at least partly desolvated through contact with gas molecules present in the atmospheric pressure region of the source. Desolvation may also be assisted by suitably directing (relative to the aerosol axis) gas flows into that region, and/or by heating the capillary, nebulizer and gas flows. Improved desolvation may also be obtained by heating the wall enclosing the atmospheric pressure region, particularly in the vicinity of the orifice through which ions may pass leave the source and pass into the mass analyser. Some prior sources also comprise means for flowing heated gas counter to the direction of travel of ions and droplets through the orifice.
Often, the solution admitted into API ionization sources is the eluent from a liquid chromatograph. Commonly used chromatographic flow rates are between 0.1 and 1.0 ml/min, but only a small fraction of the aerosol generated from the solution passes through the orifice into the region of lower pressure. The remainder of the aerosol is waste. Because the solvents (and samples) used for liquid chromatography may be poisonous, the atmospheric pressure region of an API source is usually enclosed. The chamber also serves to reduce contamination from material that may be present in the laboratory air causing interference to an analysis. As one or more flows of desolvation gas are introduced into the chamber, it must be fitted with an exhaust port through which gas and waste solvent can leave and be conducted to a safe discharge point.
Unfortunately, the majority of samples typically analysed with liquid chromatography are non-volatile, and the solvents employed often comprise non-volatile buffer salts. Because the majority of the spray does not enter the orifice, these non-volatile constituents tend to accumulate on surfaces within the atmospheric pressure enclosures, from which they may subsequently be released by contact with the aerosol to interfere with a subsequent analysis. They may also form insulating layers on electrically conducting surfaces within the chamber, which may become electrically charged and adversely affect the transport of ions into the orifice. In order to maintain performance, therefore, the atmospheric pressure chamber of prior API sources requires regular cleaning.
It is an object of the invention to provide API ionization sources and spectrometers, and apparatus for use in such ionization sources and spectrometers, in which the deposition of material on critical surfaces inside the sources is less than in prior sources. It is another object of the invention to provide API ionization sources, spectrometers and apparatus for use in such ionization sources and spectrometers that are more easily cleaned than are prior equipment. A further object of the invention is to provide apparatus for exhausting API ionization sources and spectrometers.
As used herein, “atmospheric pressure” includes the operation of an ion source in the presence of significant quantities of gas, perhaps with pressures several hundred torr either side of atmospheric pressure itself. The term is generally used in the art to distinguish this type of ionization source from those that operate under high or medium vacuum, for example, electron impact or chemical ionization sources. Further, the term “charged particles” is meant to include singly- and multiply-charged ions, solvated ions, adduct ions, and cluster ions, etc, all formed from a sample in an ionization source operating at atmospheric pressure (as defined above), and also charged droplets of solvent comprising molecules or ions characteristic of a sample.